Seems you have already figured out how this works. In case, you haven't....
Fixed Orifice Turbocharger Control
The simplest form of turbocharger control is to have a fixed orifice exhaust bypass.
A proportion of the exhaust gases always drive the turbo, from the ground upto the maximum operating altitude. This orifice is preset by maintainence guys. This preset orifice setting results in a critical altitude of 12,000ft density altitude (full throttle, 2600 RPM & MAP 40" Hg). Any higher than that, the aircraft will not be able to maintain this MAP.
Critical Altitude
The height above which maximum manifold pressure can no longer be maintained.